Advances in ceramic processing have permitted the replacement of various components of electrical and mechanical equipment with sintered ceramic parts. For example, ceramics have found widespread use in electronic components, cutting tools and, to a lesser degree, as structural substitutes for metal parts in engines and other machinery. Lightweight ceramic parts are, in fact, preferable in many applications because of their refractory properties and chemical inertness. However, the properties exhibited by ceramic materials are determined by the sintered microstructure and under present processing techniques the properties can be highly variable depending, in large part, upon the quality of the starting powder. In some industries the cost of rejection of ceramic parts at various processing stages (i.e., raw material inspection, part shaping, firing and finishing) can approach a cumulative 50 percent of the total manufacturing cost.
Typically, the process of manufacturing ceramic parts begins with the packing of a metallic oxide powder into a mold (or otherwise shaping and compressing) to form a so-called "green body" that is subsequently sintered at high temperature to yield the ceramic material.
In some applications, a porous ceramic body is desired, for example, chemically inert porous structures can serve as filter membranes, chromatography substrates, catalytic substrates and gas sensors (when appropriately doped). For such porous applications, the green bodies are typically formed at about 40 to 50 percent of their maximum density. In other applications, where structural strength is most important, the preferred density of the green body ranges from about 60 to about 70 percent so that the final ceramic part can approach its theoretical maxiumum density upon sintering.
A wide variety of prior art techniques for producing sinterable powders are known. Most conventional techniques begin with the heating (calcining) of metallic nitrates or hydroxides followed by pulverizing and further grinding to yield metallic oxide powders. Two basic problems with these techniques have been the large size distributions (i.e., from about 0.5 micron to about 10 micron) and the irregular shape of the particles. Because of these factors, orderly packing of powders into green bodies has been difficult and sintering at high temperatures (i.e., 1700.degree. C.) has been necessary. Accordingly, sintered microstructures (and properties) have not been readily controlled.
It has also been suggested that metallic oxides can be precipitated by hydrolysis of metal alkoxides from liquid solutions. See generally, Mazdiyasni et al., "Preparation of Ultra-High-Purity Submicron Refractory Oxides", Vol. 48, J. of Am. Ceramic Society, pp. 372-375 (1965). However, the particles formed by this method are extremely small (average size: 100 to 200 angstroms) and tend to agglomerate, which makes effective sintering difficult unless sintering additives are utilized.
There exists a need for better methods for making fine metallic oxide powders and the like. Preferably, the powders should be monosized (or have a very small size distribution), spherical, non-agglomerated and have an optimal size of about 0.05 to about 0.7 microns (depending upon the material and specific processing plans). Such powders would find widespread use and satisfy a variety of long-felt needs in forming ceramic parts for advanced electrical, structural, and energy conversion applications.